1. Field of The Invention
The present invention relates to circuits for eliminating noise, and more particularly to a circuit for eliminating popping sound when a computer is switched on and off.
2. Description of Related Art
In an audio system of a computer, at the time when the computer is turned on or off, a popping sound is emitted from a speaker. In order to prevent the popping sound, a conventional circuit for eliminating noise is provided as shown in FIG. 1. The noise elimination circuit includes a sound card for receiving a digital audio signal and transforming it to an analog audio signal; a power supply with a +5V voltage pin connected with the sound card for providing a working voltage thereto; and an N-channel-enhancement MOSFET Q with a gate connected with a PSON# kPower Supply ON) pin of the power supply, a drain connected to the +5V voltage pin of the power supply through a resistor R, and a source connected to ground. During powering down time, the PSON# pin of the power supply switches from low to high level. Thus, the MOSFET Q is turned on and rendered conductive. The +5V voltage pin for providing a working voltage to the sound card is connected to ground through the conductive MOSFET Q. The sound card does not work and generate any audio signal as without working voltage, thus the popping noise is nearly eliminated when powering off the power supply.
However, the circuit for eliminating noise doesn't completely eliminate the popping sound as electric charge stored in capacitors connected to the power supply is not discharged instantly when the computer is powered down and does nothing about the popping sound when the computer is powered up.
What is needed, therefore, is a circuit for completely eliminating turn-on and turn-off popping noise from a computer.